If the following code is executed MATLAB makes a mistake. Can someone verify this?
floor([0.1:0.1:2]/0.01) So what is the 129 doing here??
ans = 10 20 30 40 50 60 70 80 90 100 110 120 129 140 150 160 170 180 190 200 
3 Answers
It is a floating point rounding error because of the colon-generated vector.
Like Rasman said, if you do:
floor((0.1:0.1:2 + eps) / 0.01) There will be no rounding errors.
However, based on how the colon operator works, I suggest that you do the same calculation like this:
floor([(1:20)/10] / 0.01) [Edit: following Rasman's comment, I will add that the latter approach works for negative values as well, while adding eps sometimes fails]
The bottom line is that it is better using the colon-operator with integer numbers to minimize rounding errors.
Comments
It is probably doing a floating point calculation resulting in an inexact value of 129.99999999999999... something instead of 130. and then you floor it to 129.
Comments
it's a rounding approximation brought on by the array construction. The solution would be to add eps:
floor([0.1:0.1:2]/0.01+ eps([0.1:0.1:2]/0.01)) 
5 Comments
mmumboss
Thank you, is this a workaround for this specific situation? Or do you recommend this kind of code always in such cases?
Rasman
Most of the time I've seen a rounding problem it is because it is off by a single least significant bit (perhaps 2, although I can't recall seeing one. You can be super-safe by adding 2*eps, but for this type of problem it works:
any((floor([0.1:0.1:200]/0.01 + eps([0.1:0.1:200]/0.01))) - (10:10:20000))Rasman
I take that back, it works while you are in positive territory
Rasman
@EitanT I completely disagree, you're trying to add a value-specific eps, not some generic number. Use my example and compare:
any((floor([0:0.1:2]/0.01 +eps)) - (0:10:200)) vs any((floor([0:0.1:2]/0.01 +eps([0:0.1:2]/0.01))) - (0:10:200))
Eitan T
@Rasman +1 for beating me to the answer with
eps, and +1 for the comment on the negative values ;) About the second comment, look at what I wrote in my answer, in the first approach. Add the eps to the colon-generated vector, before the division.
10:10:200?